1. Field of the Invention
The present invention relates generally to molding plastic containers from extruded plastic parisons. More particularly, the present invention relates to tandem molding two containers in a mold.
2. Description of the Prior Art
Plastic containers for use in the food processing industry are commonly blow molded from a parison that is cut from extruded plastic tubing. The plastic tubing may include an inner circumferential layer that does not impart any taste to food when left in contact therewith an outer circumferential layer that has low permeability and so preserves freshness by excluding air and a filler layer that is disposed between the inner and outer circumferential layers. A lower cost material is used for the filler layer; and the filler layer includes scrap from the molding process.
Commonly the process of molding plastic containers comprises continuously extruding plastic tubing from an extrusion mold, cutting a parison from the extruded plastic tubing placing the parison between two halves of a blow mold, injecting a blow needle through a moil portion of the parison blowing the parison into a container that includes an integral moil removing the container and moil from the blow mold, and removing the moil from the container.
In order to increase the production rate of blow molding several blow molds may be used with a single extrusion machine. The blow molds are transported in a predetermined pattern and sequentially receive a parison that is cut from the tubing that is being extruded. In one common arrangement, the blow molds are circumferentially spaced on a vertically-disposed disk and each receive a parison as they arrive at a particular circumferential location.
Commonly each blow mold forms a single container. However the prior art also teaches using molds that include cavities for two containers.
For instance, Ruekberg, in U.S. Pat. No. 3,456,290, teaches "logged" blow molding. In "logged" blow molding, two containers are blown mouth-to-mouth, and the blow needle is inserted into a moil that is interposed between the mouths of the two containers.
"Logged" blow molding allows the use of higher extrusion rates and substantially doubles the production for a machine having a given number of blow molds. However, since the parison that is used for "logged" blow molding is approximately twice as long as a parison for molding containers singly, the curvature in the parison has resulted in uneven wall thicknesses in the blown containers.
Evely ln U.S. Pat. No. 4,650,628 of common assignee with the present invention teaches "logged" blow molding in which means is provided for straightening the parison, thereby minimizing the problem of uneven wall thicknesses in containers that are "logged."
Even with the advance of Evely, blowing two containers at a time has not been without some difficulties, and these difficulties have been attributable to temperature differentials in the parison. Since blowing two containers from a single parison substantially doubles the length of the parison, then by reason of its relatively long length, and by reason of the relatively short time between leaving the extrusion and the time that blow molding occurs, there are large temperature variations in the parison.
The temperature variations in the parison have caused blemishes in the blown containers that are cosmetically objectionable to the food processing industry.
More particularly, the blemishes have appeared in one of the two containers that are "logged." Typically, the blemishes have caused a variation in the translucency of the plastic container and have looked much like a run in a painted surface. Also, the imperfections have caused a variation in the exterior surface that feels like a wrinkle that has almost been ironed out, but not completely.
It has been hypothesized that the imperfections in "logged" containers are caused by temperature variations in the parisons; and the problem has been solved by providing a delay between blowing the later-extruded portion of the parison and blowing the earlier-extruded portion of the parison.
While this delay in blowing the later-extruded portion is effective to eliminate the afore-mentioned blemishes in the containers, the reasons for the improvement are not completely understood.
It may be that elimination of the cosmetic defects is attributable to: a reduction in the temperature of the second portion of the parison, a resultant increase in the viscosity of the material a reduction in the tendency of the higher viscosity material to stick to the surfaces of the mold cavities, and better distribution of the plastic material in the blow mold.
It may be that elimination of the cosmetic defects is attributable to: a reduction in the temperature of the second portion of the parison, a resultant increase in the viscosity of the material, an increase in the pressure that is required to expand the second portion and better distribution of the second portion inside the second container cavity because of the higher blowing pressure.
Thus, it may be that the higher viscosity of the material of the second portion of the parison, as provided by the delay in blow molding, decreases the tendency of the plastic to stick to the surfaces of the blow mold and also provides an increase in the blowing pressure that overcomes the tendency of the plastic to stick to the surfaces of the blow mold, thereby providing better distribution of the plastic material, and thereby eliminating the cosmetic defects.
However, without regard to the actual causes and effects, the present invention effectively eliminates the cosmetic defects that have attended "logged" blow molding by providing a process that includes: tandem molding of containers in a mouth-to-tail arrangement, and delaying the blow molding of a later-extruded portion of a parison with respect to blow molding an earlier-extruded portion of the parison into a container.
Stated another way, the present invention effectively eliminates the cosmetic defects that have attended "logged" blow molding by providing a process that includes: tandem molding of containers in a mouth-to-tail arrangement, and cooling the later-extruded portion of the parison during the blow molding of an earlier-extruded portion of the parison, and/or cooling the later-extruded portion of the parison subsequent to blow molding the earlier-extruded portion.